Ultrasonic inspection systems have found wide-spread utility in a variety of fields as they can detect the internal defects of an object without destruction of the object. The existence or absence of internal defects in an object is often checked over a predetermined range or area of the object. In this case, the predetermined range or area of the object is scanned by an ultrasonic wave radiated from a probe of an ultrasonic inspection system to practice the inspection of the object. As such probes, array probes composed individually of a number of piezoelectric elements arranged in a row have been actually employed. An ultrasonic inspection system making use of such an array probe will hereinafter be described.
FIG. 1 is a perspective view of a scanner unit of the conventional ultrasonic inspection system, while FIG. 2 and FIG. 3 are plan and side views of the array probe, respectively. In each figure of the drawings, there are shown a water tank 1 for conducting an inspection therein, water 2 contained in the water tank 1, and an object 3 placed on a bottom wall of the water tank 1. Designated at numeral 4 is a scanner, which is constructed of the following members: a scanner base 5 with the water tank 1 mounted thereon, frames 6 fixed on the scanner base 5, an arm 7 mounted on the frames 6, a holder 8 placed on the arm 7, a pole 9 attached to the holder 8, and the array probe designated at numeral 10. The frame 6 can drive the arm 7 along Y-axis by an unillustrated mechanism, while the arm 7 can drive the holder 8 along X-axis by a mechanism also not illustrated. Further, the holder 8 can drive the array probe 10 along Z-axis (in a direction perpendicular to X-axis and Y-axis) by a mechanism (not shown) in cooperation with the pole 9.
The array probe 10 is construction of a number of minute piezoelectric elements, which will hereinafter be called "array element oscillators", arranged in a row. The direction of their arrangement is in coincidence with X-axis. Upon application of a pulse, each array element oscillator radiates an ultrasonic wave and converts its reflection wave from the object 3 into a corresponding electrical signal. The individual array element oscillators are indicated at numerals 10.sub.1 -10.sub.n in FIG. 2 and FIG. 3. Incidentally, dots indicate sampling points, YP a sampling pitch along Y-axis, and XP a sampling pitch along X-axis. Further, AP designates the pitch between the individual array oscillators 10.sub.1 -10.sub.n. Designated at numeral 11 is a casing which accommodates the array probe 10 and the like.
The function of the array probe 10 shown in each figure described above will now be outlined with reference to FIG. 4 and FIG. 5. In FIG. 4, there are shown array element oscillators T.sub.1 -T.sub.9 arranged in a row, delay elements D.sub.1 -D.sub.9 connected to the array element oscillators T.sub.1 -T.sub.9, respectively, and a pulse p to be inputted to the individual array element oscillators T.sub.1 -T.sub.9. The delay elements D.sub.1,D.sub.9 are set to have the same delay time (t.sub.19). Similarly, the delay elements D.sub.2,D.sub.8 are set at the same delay time (t.sub.28), the delay elements D.sub.3,D.sub.7 at the same delay time (t.sub.37), and the delay elements D.sub.4,D.sub.6 at the same delay time (t.sub.46). The individual delay times so set satisfies the relationship of the following inequality: EQU t.sub.19 &lt;t.sub.28 &lt;t.sub.37 &lt;t.sub.46 &lt;t.sub.5 ( 1)
where t.sub.5 is the delay time of the delay element D.sub.5.
Now assume that the pulse p is inputted after setting the delay time of the individual delay elements D.sub.1 -D.sub.9 at predetermined values while maintaining the relationship of the inequality (1). Ultrasonic waves are then radiated from the respective array element oscillators T.sub.1 -T.sub.9 in accordance with the above-set delay times so that the ultrasonic waves from the array element oscillators T.sub.1,T.sub.9 are radiated first and the ultrasonic wave from the array element oscillators T.sub.5 is radiated last. The ultrasonic waves radiated as described above advance while spreading radially. There is a point where the maximum amplitude of vibrations of the ultrasonic wave from each array element oscillator coincides. This point is indicated by letter F in FIG. 4. Since the magnitude of the ultrasonic wave at this point F is far greater compared to the magnitudes of the ultrasonic wave at other points, a state is developed as if the ultrasonic waves from the individual array element oscillators T.sub.1 -T.sub.9 have converged at the point F as indicated by dashed lines. In other words, if suitable delays are applied to the radiation of ultrasonic waves from the array element oscillators arranged in a row, the ultrasonic waves radiated from the individual array element oscillators can be brought into a state similar to the state that these ultrasonic waves have converged at the point F. This point F is called the "focal point". Describing this further, the ultrasonic beams B which converge at the focal point F as indicated by the dashed lines are outputted by the array element oscillators T.sub.1 -T.sub.9. If the individual delay times are set smaller than their corresponding delay times described above while maintaining the relationship of the formula (1), the focal point F is caused to move to a longer focal point F' as indicated by alternate long and short dash lines (beams B'). It is, therefore, possible to choose the position of the focal point by adjusting the delay times of the delay elements D.sub.1 -D.sub.9. Application of this to the inspection of the object 3 makes it possible to choose the depth of the site to be inspected.
FIG. 5 schematically illustrates the function of the array probe 10 depicted in FIG. 2 and FIG. 3. In this figure, numerals 10.sub.1 -10.sub.n indicate the same array element oscillators as those shown in FIG. 2 and unillustrated delay elements are connected to the respective array element oscillators 10.sub.1 -10.sub.n. In the illustrated embodiment, m pieces of the array element oscillators 10.sub.1 -10.sub.m are first selected and the delay times for ultrasonic waves to be radiated from these oscillators are suitably set, whereby the ultrasonic waves apparently converge at a singe focal point as described above. These focal point and apparent ultrasonic beams are indicated by symbols F.sub.1 and B.sub.1 in FIG. 5, respectively. The array element oscillators with the numbers increased by one are next selected and, to the like number, i.e., m pieces of the array element oscillators 10.sub.2 -10.sub.m+1, delay times of the same pattern as those applied to the array element oscillators 10.sub.1 -10.sub.m in the preceding delay operation are applied. The resulting focal point is indicated at symbol F.sub.2 whereas the ultrasonic beams so radiated are indicated by symbol B.sub.2. The array element oscillators with the numbers successively increased one by one are then selected and, at the end, the array element oscillators 10.sub.n-m+1 -10.sub.n are selected and the delay times of the same pattern are applied to the array element oscillators 10.sub.n-m+1 - 10.sub.n to obtain a focal point F.sub.n-m+1 and ultrasonic beams B.sub.n-m+1. By the method as described above, the array probe 10 has performed ultrasonic scanning from the focal point F.sub.1 to the focal point F.sub.n-m+1 as a consequence. Since this scanning is performed electronically at a high speed, it will hereinafter be called "electronic scanning". Incidentally, in FIG. 5, "AP" indicates the pitch between array element oscillators while "SP" designates a sampling pitch. In the illustrated embodiment, they are equal to each other.
A description will next be made of a control unit of the ultrasonic inspection system making use of the array probe.
FIG. 6 is a block diagram of the control unit, in which there are illustrated the array probe 10 described above, motors 7M,8M for driving the arm 7 along Y-axis and the holder 8 along X-axis, respectively, and encoders 7E,8E for outputting drive signals to the corresponding motors 7M,8M and detecting and outputting their driven distances. Incidentally, the motor 8M and encoder 8E are used to position the array probe 10 at a suitable location inside the water tank 1 and do not take part in ultrasonic scanning along X-axis, said scanning being to be described later. Designated at numeral 20 is a signal processor, which is composed of a CPU (central processing unit) 20a, an image memory 20b for image processing, an interface 20c for performing input/output operation between the signal processor 20 and external circuits, a keyboard 20d, etc. Although the signal processor 20 is additionally equipped with memory devices such as RAM and ROM, their illustration is omitted. Numeral 21 indicates a display.
Designated at numeral 22 is a transmission controller which, in accordance with commands from CPU 20a, controls the delay times and the selection and change-over of array element oscillators, said delay times, selection and change-over having been described above with reference to FIG. 4 and FIG. 5. There is also illustrated pulsers 23 for outputting a pulse p. These pulsers 23 are provided corresponding to the respective array element oscillators. Numeral 24 indicates receivers for receiving reflected ultrasonic signals from the corresponding array element oscillators and then for amplifying the same. These receivers 24 are also provided corresponding to the respective array element oscillators. Designated at numeral 25 is a reception controller which performs control of the aforementioned delay, selection and change-over for signals from the respective array element oscillators. There is also illustrated a waveform adder 26 for adding all reception signals outputted at the same time as a result of the delays at the reception controller 25. Numeral 27 is a main amplifier for amplifying each output signal from the waveform adder 26. The degree of amplification by the main amplifier 27 is determined by a command from CPU 20a, which command is in turn determined based on an input from external equipment such as the keyboard 20d. Numeral 28 designates a peak detector, which is equipped with the function that only signals within a predetermined depth range are collected and only the peak value among the signals within the range is held and outputted. Designated at numeral 28 is an A/D converter for converting the peak value, which has been held in the peak detector 28, into its corresponding digital value. Operation of the control unit will be described with reference to FIG. 7.
FIG. 7 is a more detailed block diagram of the control unit so that the description of the operation of the control unit shown in FIG. 6 can be facilitated. In FIG. 7, elements either identical or equivalent to those depicted in FIG. 6 are identified by like reference numerals or symbols. Further, illustration of the motors 7M,8M, encoders 7E,8E and keyboard 20d are omitted. In addition, two array probes are shown. This is to mean that the array probe is a separated transmit-receive array probe with one of the array probes 10 being for transmission and the other for reception. Such an array probe is employed with a view toward improving the resolution in the depthwise direction. Incidentally, in the case of a combined transmit-receive array probe of the type that the same array element oscillators are used for both transmission and reception, the construction and operation of the control unit are also the same as in the case of the separated transmit-receive array probe.
FIG. 7 also illustrates a clock pulse generator 22a, a transmit beam converging circuit 22b and a transmission-side matrix switching circuit network 22c. They make up the transmission controller 22 depicted in FIG. 6. Further, a receiving beam converging circuit 25,26 constitutes a part of the reception controller 25 and the waveform adder 26, both depicted in FIG. 6.
Now assume that transmission and reception of a single ultrasonic beam is performed by eight array element oscillators. The transmit beam converging circuit 22b outputs in a predetermined delay pattern signals, which are to be applied to eight pulse generators out of the pulse generators 23, on the basis of a clock pulse from the clock pulse generator 22a. Pursuant to a command from CPU 20a, the transmission-side matrix switching circuit network 22c determines to which array element oscillators, in other words, to which pulse generators the eight signals outputted in the predetermined delay pattern from the transmit beam converging circuit 22b should be applied, and performs switching in accordance with the selection. At the pulse generators 23, signals are outputted in the above-described predetermined delay pattern from the above-selected eight consecutive pulse generators, whereby the array element oscillators connected to these pulse generators, respectively, are excited to output the desired ultrasonic beam.
On the other hand, the array element oscillators on the reception side receive the ultrasonic wave so that signals corresponding to the ultrasonic wave are outputted to the corresponding preamplifiers of the preamplifiers 24, said corresponding preamplifiers being connected to the eight array element oscillators. These signals are therefore amplified. In accordance with a command from CPU 20a, the reception-side matrix switching circuit network 25 feeds the signals, which have been amplified by the respective preamplifiers, one by one to the corresponding ones of the delay elements which are contained in the received beam converging circuit 25,26 and create delay patterns. As a consequence, the eight signals so received are outputted at the same timing in the output stages from the corresponding delay elements of the received beam converging circuit 25,26 and are then added together.
At the main amplifier 27, the signal so added is amplified in accordance with an amplification degree set by a command from CPU 20a. The signal so amplified is delivered through the peak detector 28 and A/D converter 29 so that it is converted to a digital value. The value so converted is then stored at an address of the image memory 20b, said address having been determined as a result of an address arithmetic operation at CPU 20a.
The operation of transmission and reception of a single ultrasonic beam by eight array element oscillators has been described above. By suitably switching and controlling the transmission-side matrix switching circuit network 22c and the reception-side matrix switching circuit network 25, the individual array element oscillators 10.sub.1 -10.sub.n of the array probe 10 can be selected in groups, each consisting of eight array element oscillators, while successively increasing the numbers of the eight array element oscillators one by one. As a consequence, electronic scanning along X-axis can be carried out. Upon completion of the electronic scanning, the motor 7M is driven in accordance with a command from CPU 20a so that the arm 7, namely, the array probe 10 is shifted by the predetermined sampling pitch YP along Y-axis (mechanical scanning). In this state, electronic scanning of a second row, as viewed along X-axis, by ultrasonic wave is performed in a manner similar to the electronic scanning described above. By repeating such an operation, ultrasonic scanning of the X-Y plane of the object 3 can be performed. During the ultrasonic scanning, signals of reflected waves received at the individual receivers 24 are successively stored at prescribed addresses, respectively, in the image memory 20b. Based on the data stored in the image memory 20b, an ultrasonic image of the object 3 by the above ultrasonic scanning is shown on the display 21. Checking of any defects in the object 3 can be conducted by observing the ultrasonic image so displayed.
The scanning time in each single line along X-axis is extremely short so that the mechanical scanning along Y-axis can be performed without interruption in the course of the inspection. Further, the number of array element oscillators in each group to be selected can be set as desired.
More detailed construction of the transmit beam converging circuit 22b, transmission-side matrix switching circuit network 22c, pulse generators 23, preamplifiers 24, reception matrix switching circuit network 25 and received beam converging circuit 25,26, all illustrated in FIG. 7, are disclosed in Japanese Patent Application Laid-Open (Kokai) No. HEI 2-69654.
The conventional ultrasonic inspection system described above can promptly and accurately inspect the existence or absence of defects in the object 3. It is, however, to be noted that the signal level may scatter from one data sampling to another when the data sampling is conducted at individual focal points in electronic scanning (i.e., scanning along X-axis) by the above-described conventional ultrasonic inspection system. This is shown in FIG. 8, in which sampling points in the direction of electronic scanning are plotted along the axis of abscissas while the levels of received signals are plotted along the axis of ordinates. As is clearly envisaged from the figure, the levels of received signals scatter to a substantial extent. The present inventors conducted an investigation to determine possible causes for the occurrence of such scattering. As a result, it has been found that its primary cause resides in the array probe 10, pulser 23 and receiver 24. This will next be described with reference to drawings.
FIG. 9 is a detailed block diagram of the array probe, pulser and receiver, in which numerals 10.sub.1 -10.sub.n indicate the individual array element oscillators constituting the array probe 10. As has been described above, the individual array element oscillators 10.sub.1 -10.sub.n are connected to corresponding pulsers 23.sub.1 -23.sub.n and receivers 24.sub.1 -24.sub.n. Each single channel for transmitting and receiving a single ultrasonic beam is constructed by the combination of one of the array element oscillators, its corresponding pulser and receiver, and plural conductors connecting them. In such a construction, the existence of certain differences in sensitivity among the individual array element oscillators 10.sub.1 -10.sub.n, pulsers 23.sub.1 -23.sub.n and receivers 24.sub.1 -24.sub.n cannot be avoided. If an array element oscillator, a pulser and a receiver, all having low sensitivity, are connected, the level of a signal to be received through the resulting channel will be appreciably reduced. If an array element oscillator, a pulser and a receiver, all having high sensitivity, are connected conversely, the level of a signal to be received through the resulting channel will be appreciably higher. As a result, the levels of signals received through the individual channels scatter.
FIG. 10 illustrates inspection of a defect-containing object by an ultrasonic inspection system with such scattering in received signals as described above, while FIG. 11 shows an ultrasonic image obtained as a result of the inspection depicted in FIG. 10. FIG. 10 shows the object 3, a concaved defective portion 3f of the object 3, and the array probe 10. On the other hand, FIG. 11 illustrates a screen 21a of the display 21, an ultrasonic image A of the contour of the object 3, and an ultrasonic image 3f' of the defective portion 3f. Symbols G.sub.1 -G.sub.6 indicate stripes occurring along Y-axis as a result of the scattering of signals received through the individual channels in the ultrasonic inspection system (in practice, a number of stripes of various widths occur). Upon observation of the ultrasonic image, these stripes make it difficult to watch the overall image and, hence, to discover the defective portion. Where the difference in level between ultrasonic signals from the defective portion and those from a defect-free portion is very small, for example, inconvenience is caused because their boundary becomes unclear due to interference by the stripes.
As a method for eliminating such scattering, one could consider providing the individual pulsers 23.sub.1 -23.sub.n or receivers 24.sub.1 -24.sub.n with sensitivity adjusters, respectively, so that the levels of received signals can be controlled. This method may be feasible where the total number n of the array element oscillators is small. Where the total number n ranges from 100 to 200, an extremely long time is required for the sensitivity adjustment alone and, in addition, any attempt to automate the sensitivity adjustment for shortening the time required therefor leads to the need for an enormous circuit-loading area and their control becomes extremely complex. Moreover, even if such sensitivity adjusters are provided, the entire sensitivity adjustment must be conducted again when the array probe is replaced (such a replacement is done frequently). In addition, the following problem still exists even if the above sensitivity adjusters are provided. Namely, the cause for scattering in sensitivity also exists in the reception controller 25 provided next to the receiver 24. Since the reception controller 25 has plural input/output signal lines, the intensity of an ultrasonic signal varies depending on which one of the signal lines the ultrasonic signal passes through. Because of this, even if scattering in sensitivity at the stage of the pulsers or receivers is eliminated completely, scattering still occurs as a result of the passage of signals through subsequent signal lines.
An object of this invention is overcome the above problems of the conventional art and, hence, to provide an ultrasonic inspection system which can easily eliminate the influence of the scattering of received signals and can display a clear image of a defective portion to permit accurate inspection of an object.